Optical transmission cell with minimized spurious absorption

Abstract

A spectroscopic measuring device for minimizing spurious absorption due to undesired gases. The device includes a probe body, or a transmission cell formed from a central measurement cell and first and second probe bodies. Each probe body is subject to leakage of undesired gas, especially over time in the presence of high pressure gas. Each probe body includes a bore located between a primary window disposed at or near a distal end and a secondary window located at or near the proximal end. It being observed that the absorbance is proportional to the pathlength and inversely proportional to the volume as long as the pressure in the probe body remains low compared to that in the measurement cell, a glass filler rod is located in the bore and a is void located adjacent to the filler glass rod, thereby minimizing spurious absorption even in the presence of leakage.

Claims

1. A spectroscopic measuring device for minimizing the effect of undesired gases that that interferes with measurement, comprising: a central section for receiving a gas passing through the central section to be analyzed; first and second probe bodies that are inherently subject to leakage of undesired gas, wherein the first and second probe bodies are attached to the central section with one injecting light into the central section and one collecting light from the central section in order to measure a characteristic of the gas passing through the central section, and wherein each probe body comprises: an axis, a distal end, a proximal end, and an interior; a primary optical window disposed at or near the distal end of the probe body and sealed with respect to the probe body; a secondary optical window located at or near the proximal end of the probe body and sealed with respect to the probe body, a bore in the interior of the probe body that defines an optical path between the primary optical window and the secondary optical window, the bore having a nominal length and a nominal volume; a filler rod located in the bore and capable of transmitting light entering the primary optical window along the nominal length of the bore; and a void in fluid communication with the bore and located adjacent to the filler rod and outside of the optical path, the void increasing the nominal volume of the bore and providing a location for holding undesired gas outside of the optical path; wherein each filler rod fits tightly within a first portion of the bore and wherein each void comprises an expanded space in a second portion of the bore.

2. The spectroscopic measuring device of claim 1, wherein the primary optical window of each probe body is sealed with respect to the respective probe body by a high pressure seal comprising a C-ring.

3. The spectroscopic measuring device of claim 1 wherein each filler rod is a glass rod.

4. The spectroscopic measuring device of claim 1 wherein each void comprises an expanded space around a portion of the filler rod.

5. The spectroscopic measuring device of claim 1 wherein the first portion is a distal portion of the bore and wherein in the second portion is a proximal portion of the bore.

6. The spectroscopic measuring device of claim 5 wherein each bore has a first diameter in the distal portion and a second larger diameter in the proximal portion, the void being formed by the second larger diameter in the proximal portion.

7. The spectroscopic measuring device of claim 1 wherein the central section comprises a cross fitting.

8. The spectroscopic measuring device of claim 1 wherein the light comprises near-infrared radiation.

Description

DESCRIPTION OF THE DRAWINGS

(1) The preferred embodiments of the just summarized invention can be best understood in connection with a detailed description of the following figures.

(2) FIG. 1 is a schematic cross-sectional view of a transmission cell 110 of a first construction having a centrally located cell body 20, first probe 40 with a bore 47, and second probe 50 with a bore 57, and where the bores 47, 57 of the first and second probes 40, 50 (Regions 1 and 3) are unfilled and subject to spurious absorption due to the presence of undesired gasses therein;

(3) FIG. 1a is a simplified portion of the first probe 40 from FIG. 1 showing that its bore 47 has a length Lp and a diameter Dp, a glass rod 48 that fits tightly within the bore 47, and the resulting assembly which has virtually no unfilled volume;

(4) FIG. 2 is a schematic cross-sectional view of a transmission cell 210 of a second modified construction where the reference numbers to items similar to corresponding items in FIG. 1 have been omitted for clarity, and where the bores 47, 57 of the first and second probes 40, 50 (Regions 1 and 3) have been modified to contain filler rods 48, 49, and to have unfilled voids 49, 59 of increased volume; and

(5) FIG. 2a is a simplified portion of the first probe 40 from FIG. 2 showing that its bore 47 has a length Lp and two diameters, a first diameter Dp1 at a distal portion of the bore 47 and an increased diameter Dp2 along a portion of its length at a proximal portion of the bore 47, a glass rod 48 that fits within the bore 47, and the resulting assembly which includes a void or unfilled volume 49.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) To better understand the preferred embodiments of the invention that resolve the spurious absorption problem that we have encountered, we need to introduce the concept of Beer's Law. This states that the optical absorption by a material can be specified in terms of an Absorbance (A) which is proportional to concentration of the material (C) and the pathlength (L). i.e.
A.sub.c=a.sub.cL.sub.cC.sub.c, where the subscript c refers to the cell.Eq. 1

(7) Here, a.sub.c is an absorption coefficient for the material in the cell.

(8) A similar expression, except with subscript c replaced by subscript p would apply to material in the probe body.

(9) The transmission through a volume is related to the Absorbance by the following expression:
T=log.sub.10A.Eq. 2

(10) In many applications, such as natural gas analysis, the vapor in the cell body will be at a high pressure. The pressure is related to concentration by the ideal gas law:
P=CRT,Eq. 3

(11) where, R=the gas constant, and T=temperature.

(12) Here, the concentration is given by C=n/V, where, n=number of moles of the gas and V=volume. For a given pressure differential, DP, between the cell and the probe body, the rate of flow from the cell to the probe, F, can be stated as:
F=KDP,Eq. 4
where K is a constant that takes into consideration the cross sectional area and the leakage characteristics of the window seal.

(13) As long as temperature is constant and the pressure in the probe body is very low compared to that in the cell, DP will be constant and the gas pressure in the probe will build up linearly over time:
P.sub.p(t)=(KDP/V.sub.p)t,Eq. 5
Where Vp is the volume of the probe body.

(14) The concentration will be:
C.sub.p(t)=P.sub.p(t)/RT=(KDP/V.sub.p)t, where K=K/RTEq. 6

(15) The optical absorbance will be given by the expression
A.sub.p(t)=a.sub.pL.sub.pC.sub.p(t)=a.sub.pL.sub.p(KDP/V.sub.p)t=a.sub.pKDP(L.sub.p/V.sub.p)t.Eq. 7

(16) In other words, the absorbance is proportional to the pathlength and inversely proportional to the volume as long as the pressure in the probe remains low compared to that in the measurement cell.

(17) Even though the leak rates through the coated metal seals are usually very low, the buildup of small amounts of gas in the probe bodies over time can lead to measurement problems. This is due to the sensitivity of the measurements to small changes in the near-infrared spectrum.

(18) With the foregoing observations in hand, one can see how the embodiment of FIG. 2 offers a significant solution to the problem of spurious absorption. In particular, the second transmission cell system 210 of FIG. 2 substantially reduces the effects of gas leakage into the probe bodies 40, 50 by minimizing the ratio of pathlength to probe volume, i.e. (L.sub.p/V.sub.p). The first step is to place a glass rod 48, 58 in the probe body 40, 50) so that most of the optical path is within the rod 48, 58 rather than in to open volume of the probe 40, 50. The simplest way to do this is to slip a tight-fitting rod 48, 58 into the bore 47, 57 of the previously existing probes. However, the use of a filler rod 48, 58 does not by itself solve the problem since it reduces the volume of the bore 47, 57 almost as much as the pathlength. i.e. for a tight fitting rod, the unfilled volume will be
V.sub.u=(pD.sub.p.sup.2/4)L.sub.u,Eq. 8

(19) Where L.sub.u is the length of the unfilled volume 49 and, again, D.sub.p is the inner diameter of the probe. The ratio of pathlength to cell volume is
Lu/Vu=4/D.sub.p.sup.2

(20) Thus, substituting Eq. 9 into Eq. 7, we see that the Absorbance is independent of pathlength.

(21) Our solution to this problem is to increase the volume of the probe that lies outside of the glass rod. This can be done within the mechanical constraints of the probe design by increasing the bore diameter for a portion of its length.

(22) The design illustrated in FIG. 2 increases the probe volume by about two orders of magnitude compared to the volume between the ends of the rod and the probe windows. It should thus reduce the absorbance by two orders of magnitude. The glass rod is indicated by 48 or 58. The increased volume provided by discrete or annular voids located adjacent to the filler rod 48 or 58 is indicated by 49 or 59.

(23) Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiments have been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. The claims are thus to be understood to include the specifically illustrated and described embodiments, structures based on equivalents concepts, and substitutions that incorporates the invention. For example, the probe volume can also be increased by creating an additional volume away from the main body of the probe. This can be done, for example, by providing a significant length of tubing between the probe bore 47, 57 and the valve used for sealing the probe.